1. Field of the Invention
The present invention relates to frequency agile magnetrons, and more particularly, to a novel low-torque tuning mechanism for changing the frequency of a magnetron.
2. Description of Related Art
Crossed-field tubes, such as magnetrons, are commonly used to generate RF or microwave electromagnetic energy for assorted applications including radar. The magnetron commonly has a cylindrically shaped cathode coaxially disposed so as to be surrounded by a plurality of radially extending anode vanes. The space between the cathode surface and the anode provides a cavity, and a potential is applied between the cathode and the anode forming an electric field in the cavity. A magnetic field is also provided in the cavity perpendicular to the electric field. Electrons are emitted thermionically from the cathode surface and are caused to orbit around the cathode in the cavity due to the magnetic field, during which they interact with an RF wave moving on the anode. The electrons give off energy to the moving RF wave, thus producing a high power microwave output signal.
It is useful to provide a magnetron having a frequency which can be periodically changed or tuned, known as a "frequency agile" magnetron. Frequency agile magnetrons produce an output signal which is less susceptible to jamming, and which provides a higher quality radar image due to the assortment of microwave wavelengths emitted. Many techniques are used for tuning magnetrons, and typically employ changes in the capacitance or the inductance of the magnetron cavity. An example of a prior art tuning device for a coaxial magnetron is found in U.S. Pat. No. 4,531,104 for TUNABLE MAGNETRON OF THE COAXIAL-VACUUM TYPE, which issued Jul. 23, 1985 by Schaeffer.
Another prior art magnetron tuning technique involves the insertion of rotatable dielectric paddles into the cavity. The paddles have a generally planar surface which is caused to rotate by interaction with a high speed gear train driven by an external motor. The instantaneous position of the paddles relative to the electric field effects the frequency of the magnetron. When the planar surface of the paddles is generally parallel to the electric field E, as illustrated in FIG. 1a, the magnetron frequency is a minimum. Conversely, when the planar surface of the paddles is generally perpendicular to the electric field E, as illustrated in FIG. 1b, the magnetron frequency is a maximum. As the paddles rotate within the cavity, the magnetron frequency alternates sinusoidally between the minimum and maximum value, as illustrated in FIG. 2. For each full rotation of the paddles, two complete cycles of the magnetron tuning range are achieved.
This type of magnetron tuning has numerous advantages in achieving frequency agility. Since the tuner mechanism is a rotating device, the required motor power can be kept to a minimum since it must only supply enough power to overcome windage and frictional losses once normal rotational speeds are reached. Moreover, the doubling effect of the tuning cycle provides that the motor and gear train only have to rotate at half the frequency of other tuning mechanisms.
However, the mechanical rotation of the dielectric paddles within the cavity presents additional problems which reduce the operational life of the magnetrons. The gear train assembly creates a certain amount of debris consisting of a complex mixture of lubricants, metal dust, and the pressurizing gas. This debris can be deposited on the surface of the dielectric paddles and drastically alter magnetron operation. First, the debris changes the dielectric properties of the paddles and alters the tuned range of frequencies produced by the magnetron. Second, the risk of arcing between the paddles and the cathode is increased due to the reduced voltage standoff capability of the paddles. Third, there is reduced power output or excessive power variation from the magnetron by the debris changing the resonant characteristics of the anode cavity. Finally, in certain catastrophic cases, moding or missing pulses caused by changes in anode cavity properties can result.
Alternative methods of magnetron tuning which attempt to avoid the problems experienced with the method discussed above, have included the use of a moveable end plate within the cavity which changes the resonant characteristics of the cavity in order to change the inductance of the cavity. The end plate is connected via a rod to a crankshaft arrangement which translates rotational motion of a motor into axial movement of the end plate. Reciprocating motion of the end plate can sinusoidally tune the magnetron frequency. However, this technique shares many of the drawbacks of the prior technique in that it relies heavily on rigid mechanical linkages which require lubrication and generate debris. The mechanical linkages tend to wear over the life of the magnetron, which alters the tuned frequency range. Further, increasing amounts of torque must be produced by the motor to overcome the degradation of the mechanical linkages, which decreases the operational life of the motor.
Accordingly, it would be desirable to provide a technique for magnetron tuning that provides reliable operation without the generation of undesired debris within the anode cavity or the need for excessive torque to operate the moveable tuning member.